1. Field of the Invention
This invention relates generally to nuclear reactors and, more particularly, to a method and apparatus for stiffening a riser brace that lends lateral support to a jet pump assembly of a boiling water reactor.
2. Related Art
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically-shaped shroud.
FIG. 1 is a schematic, partial cross sectional view, with parts cut away, of a reactor pressure vessel (RPV) 20 for a boiling water reactor. RPV 20 has a generally cylindrical-shape and is closed at one end by a bottom head and at its other end by removable top head (not shown). A top guide (not shown) is situated above a core plate 22 within RPV 20. A shroud 24 surrounds core plate 22 and is supported by a shroud support structure 26. A downcomer annulus 28 is formed between shroud 24 and sidewall 30 of RPV 20.
An annulet nozzle 32 extends through sidewall 30 of RPV 20 and is coupled to a jet pump assembly 34. Jet pump assembly 34 may include a thermal sleeve 36 which extends through nozzle 32, a lower elbow (only partially visible in FIG. 1), and a riser pipe 38. Thermal sleeve 36 is secured at a first end (not shown) to a second end of the lower elbow. The first end of thermal sleeve 36 is welded to the second end of the lower elbow. A first end of the lower elbow similarly secured, or welded, to one end of riser pipe 38. Riser pipe 38 extends between and substantially parallel to shroud 24 and sidewall 30. A riser brace assembly 40 stabilizes riser pipe 38 within RPV 20. The riser brace assembly 40 may be fabricated of type 304 stainless steel which, after periods of use, is susceptible to cracking at welded joints. The riser brace assembly 40 is connected between riser pipe 38 and sidewall 30.
FIG. 2 illustrates the riser brace assembly 40 of FIG. 1 in further detail. Riser brace assembly 40 primarily provides lateral support to the jet pump assembly 34 via riser pipe 38, and includes a riser brace block 43 and two riser brace leaves, an upper riser brace leaf 41 and a lower riser brace leaf 42. Leaves 41 and 42 are attached to riser brace block 43 by welds, and riser brace block 43 is welded to a support pad 130 which in turn is affixed to RPV sidewall 30. At the other end, leaves 41 and 42 of the riser brace assembly 40 are connected to a yoke such as brace plate 49. Brace plate 49 in turn is welded to the riser pipe 38.
The riser brace assembly 40 is designed to accommodate the differential thermal expansion that results from reactor start-up and heat-up, and to accommodate the flow-induced vibration that is incumbent in the reactor water recirculation system (not shown) due to reactor recirculation pumps. Accordingly, a concern with the riser brace assembly 40 is that the natural frequency of the riser brace assembly 40 be greater than the vane passing frequency of the recirculation pumps at any pump speed. If the vane passing frequency of the recirculation pumps equals or exceeds the natural frequency of the riser brace assembly 40, the riser brace assembly 40 goes into resonance. The resonating riser brace assembly 40 could potentially cause the riser pipe 38 to become unstabilized, adversely affecting the jet pump assembly 34.
The present invention provides a method and clamp apparatus for stiffening a riser brace assembly so as increase the natural vibration frequency (e.g., first harmonic natural frequency) of the riser brace assembly above the vane passing frequency of a recirculation pump frequency at any pump speed, Installation of the vibration mitigation clamp apparatus should not require removal of any installed reactor hardware, such as jet pump assembly hardware or shroud repair hardware.
In an embodiment, the vibration mitigation clamp apparatus may include a first plate, a second plate and a wedge assembly. The vibration mitigation clamp apparatus is attached to upper and lower riser brace leaves of the riser brace assembly, at a location near a weld that attaches the leaves to a riser brace block of the riser brace assembly that is affixed to an RPV sidewall. The wedge assembly is expandable to apply forces on inside surfaces of the riser brace leaves, countering clamping forces applied to the first and second plate to fixedly secure the vibration mitigation clamp apparatus on the riser brace assembly.